Ultrasonic energy has been considered for tissue ablation and fragmentation of plaque and thrombosis for removal of intravascular occlusions due to atherosclerotic plaque and intravascular blood clots. Surgical devices utilizing ultrasonic probes for generation and transmission of ultrasonic energy, have been disclosed in the art (U.S. Pat. Nos. 5,112,300; 5,180,363; 4,989,583; 4,931,047; 4,922,902; and 3,805,787). Typically, the energy produced by an ultrasonic probe is in the form of very intense, high frequency sound vibrations, results in fragmentation of tissue (plaque and thrombosis) either as a result of mechanical action thereon or “cavitation” thereof, in which high energy ultrasound frequency applied to liquids generates vapor-filled microbubbles or “cavities” with the concomitant rapid expansion and collapse of the cavities that is accompanied by intense localized hydraulic shock, that causes fragmentation or dissolution of the tissue. Medical applications for ultrasonic probes providing cavitation include surgical procedures for ablation of tissues, for example, treatment of cancer, tissue remodeling, liposuction, and removal of vascular occlusions. Typically, ultrasonic probes described in the art for use in surgical procedures include a mechanism for irrigating an area where the ultrasonic treatment is being performed (e.g., a body cavity or lumen) to wash tissue debris from the area, and may further include an aspiration means to remove irrigation fluid and tissue debris from the site of the procedure. Mechanisms used for irrigation or aspiration described in the art are generally structured such that they increase the overall cross-sectional profile of the probe, by including inner and outer concentric lumens proximal to or within the probe to provide irrigation and aspiration channels. In addition to making the probe more invasive, prior art probes may also maintain a strict orientation of the aspiration and the irrigation mechanism, such that the inner and outer lumens for irrigation and aspiration remain in a fixed position relative to one another, which is generally closely adjacent the area of treatment. Thus, the irrigation lumen would not extend beyond the suction lumen (i.e., there is no movement of the lumens relative to one another) and any aspiration would be limited to picking up fluid and/or tissue remnants within the defined distance between the two lumens.
Ultrasonic probes described in the art for tissue ablation suffer from a number of limitations. Such probes depend on longitudinal vibration of the ultrasonic member comprising the probe i.e. vibration of the probe in the direction of the longitudinal probe axis to effect tissue fragmentation. Probe action in this modality therefore depends primarily on mechanical and thermal action of the probe tip for disrupting tissue, since the cavitational energy emanating from the tip, especially in narrow diameter probes such as those used to remove vascular occlusions, is minimal due to the small surface area of the tip itself. This primary mode of action imposes the following limitations on probe efficiency:
i) tissue ablation is restricted to very small area defined by the surface area of the probe tip, thereby necessitating time consuming surgical procedures to remove relatively large occluded areas with blood vessels in comparison to instruments which excise tissue by mechanical cutting, electrocautery, or cryoexcision methods.
ii) occurrence of late restenosis (typically within three months), and to a lesser extent acute re-occlusion after coronary angioplasty are major clinical problems limiting the long-term efficacy of ultrasonic surgical procedures for treatment of atherosclerosis and coronary angioplasty. While the pathogenosis of restenosis is still unclear, it has been demonstrated from autopsy specimens from patients with restenosis the pathophysiologic process leading to acute occlusion after coronary angioplasty is related either to a thrombotic mechanism or to major plaque dissection and superimposed thrombosis, and that these events leading to chronic restenosis involves vascular injury, platelet deposition and thrombosis and connective tissue synthesis. Such post operative processes are typically result from localized trauma at the surgical site caused by mechanical and thermal action of longitudinally vibrating probes.
Attempts to reduce some of the aforementioned problems associated with longitudinally vibrating probes have been disclosed in the art, wherein the primary action of the probe through longitudinal vibration is supplemented by a limited, supplementary transverse vibration of the probe tip i.e. perpendicular to the longitudinal axis of the probe. It is proposed that such secondary transverse vibrations in these probes will result in increased efficiency for surgical procedures. For example, U.S. Pat. No. 4,961,424 to Kubota, et al. discloses an ultrasonic treatment device that produces both a longitudinal and transverse motion at the tip of the probe. The Kubota, et al. device, however, still relies solely on the tip of the probe to act as a working surface. Thus, while destruction of tissue in proximity to the tip of the probe is more efficient, tissue destruction is still predominantly limited to the area in the immediate vicinity at the tip of the probe. U.S. Pat. No. 4,504,264 to Kelman discloses an ultrasonic treatment device, which improves the speed of ultrasonic tissue removal by oscillating the tip of the probe in addition to relying on longitudinal vibrations. Although tissue destruction at the tip of the device is more efficient, the tissue destroying effect of the probe is still limited to the tip of the probe. Both probes described in Kubota, et al., and Kelman, et al., are further limited in that the energy produced at the tip of the probe is unfocused, the action of the probe tends to push the tissue debris ahead of the probe tip. Likewise, the concentration of energy solely at the probe tip results in heating of the probe tip, which can create tissue necrosis, thereby complicating the surgical procedure and potentially compromising the recovery of the patient. Furthermore, such probes do not eliminate the problems associated with longitudinally vibrating probes.
The aforementioned limitations associated with longitudinally vibrating probes can be overcome entirely by utilizing an ultrasonic probe that vibrates exclusively in the transverse mode. Such probes are capable of generating substantially higher cavitational energy through a plurality of anti-nodes along the entire longitudinal axis of the vibrating probe, thereby eliminating the need for mechanical and thermal action at the probe tip. The advancing probe tip can therefore be shielded to prevent mechanical injury to the walls of the blood vessel for example, thereby precluding scarring, platelet deposition and clotting that lead to restenosis. Additionally, such probes are capable of tissue fragmentation over greater surface area (along the entire longitudinal axis) resulting in high efficiency, thus allowing for rapid surgical procedures and substantially eliminating thermal effects on tissue caused by prolonged probe operation.
Since probe vibrating exclusively in a transverse mode is entirely dependent on cavitational energy for their action, important factors for maintaining efficiency of such probes are (i) narrow probe diameter to facilitate oscillation at lower ultrasonic energies and (ii) increased longitudinal axis (probe length) that results in more cavitation anti-nodes. Although narrow probe diameters are advantages especially for negotiation through narrow blood vessels and occluded arteries, the utilization of such probes have been precluded by inability to effectively control the vibrational amplitude of thin probes, that result in potential damage to the probe and greater risk, of tissue damage resulting from their use. The use of narrow diameter probes have been disclosed in the art for providing greater maneuverability ease of insertion in narrow blood vessels. U.S. Pat. No. 4,920,954 to Allinger discloses a narrow diameter ultrasonic device wherein a rigid sleeve is used to prevent transverse vibrations U.S. Pat. No. 5,380,274 discloses a narrow diameter probe for improved longitudinal vibration having a sheath to inhibit transverse vibration U.S. Pat. No. 5,469,853 to Law discloses a thin, longitudinally vibrating ultrasonic device with a bendable sheath that facilitates directing the probe within narrow blood vessels. While the prior art has focused on the need for using sheaths on thin ultrasonic devices, their use has been entirely to prevent transverse, vibrations of the device and to protect such devices from damage resulting from such vibrations
Based on the aforementioned limitations of ultrasonic probes in the art, there is a need for ultrasonic probe functioning in a transverse mode that further obviates the shortcomings of that further overcomes limitations imposed by of narrow diameter requirements for efficient operation of such probes for rapid tissue ablation. Transversely vibrating ultrasonic probes for tissue ablation are described in the Applicant's co-pending provisional applications U.S. Ser. Nos. 60/178,901 and 60/225,060, and 09/776,015 which further describe the design parameters for such a probe its use in ultrasonic devices for tissue ablation. The entirety of these applications are herein incorporated by reference.
There is a further need for controlling the for procedures which require precise delivery of cavitation energy to defined locations, to be able to restrict the cavitation energy emanating circumferentially from a transversely vibrating p at multiple anti-nodes wastes a portion of the energy given off by the probe, as the energy is unfocused and dispensed along the length of the probe.
There is also a need in the art for a means of focussing the cavitational energy emitted by such a probe to deliver the energy to exactly to the desired location within a blood vessel while shielding the surrounding tissue from damage.